4. Conclusion and outlook Scrubbers clean ship exhaust gases by bringing the gas stream into contact with water in order to absorb solid, liquid and gaseous compo- nents like PAC in the water. The absorbed components occur as dis- solved or particulate pollutants in the discharge water which leads to their transfer into marine ecosystems. The P 71 PAH concentrations in the discharge water samples of the four ships covered a broad range in both OL and CL operation, which is explained by the differences in the engine and scrubber specifications but in particular by the quality of the fuel used on board. The P 71 PAH concentrations were three to four times higher than the P EPA PAH concentrations. Unsubstituted and alkylated naphthalenes and phenanthrenes, followed by fluorenes and pyrenes, dominated the pattern. In general CL-discharge waters showed higher PAH concentrations than OL-discharge waters, however, lower emission factors were ob- tained for CL-discharge, attributable to the loss of naphthalenes due to the intensive recirculation and due to the removal efficiency in the water treatment in CL operation. OL-discharge water is characterized by a higher dissolved fraction where naphthalenes dominate, while CL- discharge water is characterized by the particulate fraction where phenanthrenes dominate and HMW PAH are more often detected. A comparison of the PAH distribution patterns in the discharge water samples with those of the fuels indicates that the main source of PAH in discharge water is unburnt fuel (petrogenic source). Hence, the use of low-PAH fuels and more efficient engines would be expected to signif- icantly reduce ship emissions of PAH. Non-target analysis of NSO-PAC generally showed low concentra- tions in both the fuels and in the scrubber water. By use of laser- ionization non-target analysis, alkylated derivatives of 4H-cyclopenta [4,5-def]phenanthrene and/or phenylnaphthalene were for the first time tentatively identified in scrubber discharge water. A correlation between the dioxin-like effects and the PAC content of the dissolved fraction of the water samples was demonstrated. CL- discharge waters showed stronger dioxin-like effects than OL- discharge waters, although the PAC contents differ insignificantly from each other. This was partly attributed to the differences in the PAC distribution. This study provides valuable science-based insights to the ongoing discussions regarding environmental concerns from the release of scrubber discharge water into the sea. It underlines the importance of PAC to the extent possible beyond the common limitation to the 16 EPA PAH, for a more comprehensive evaluation of the environmental im- pacts of scrubber discharges. Although toxicity data for non-EPA PAH are scarce, no-effect concentrations should be predictable using models based on the chemical structure and properties of the compounds of interest. Studies on the toxicity of such PAH should be initiated. Supplementary data to this article can be found online at https://doi. org/10.1016/j.marpolbul.2024.116790. CRediT authorship contribution statement Christine Achten: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Conceptualization. OctavioMarin-Enriquez:Writing – review& editing, Writing – original draft, Visualization, Project administration, Investigation, Conceptuali- zation. Brigitte Behrends: Writing – review & editing, Writing – orig- inal draft, Investigation, Conceptualization. Sandra Kupich: Data curation. Andreas Lutter:Data curation.Richard Korth:Data curation. Jan T. Andersson:Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Conceptualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgement The authors acknowledge a grant from the German Environment Agency (UBA), Project No. (FKZ) 3719 57 101 0. We would like to ex- press our gratitude to the ship owners and ship personnel for their support, transparency and making the sampling campaign possible in the first place. We thank Dr. Martina Fenske (Federal Institute of Hy- drology (BfG), Germany) for her ecotoxicological advice and for the provision of the Yeast Dioxin Screen (YDS) data. References Abel, J., Haarmann-Stemmann, T., 2010. An introduction to the molecular basics of aryl hydrocarbon receptor biology. Biol. 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